System for controlling the rotational speed of a micromotor for use in medical instruments

ABSTRACT

A system for controlling the rotational speed of a micromotor for use in medical instruments such as a dental handpiece comprises a first transistor connected in series between said motor and a source of electric energy; a second transistor Darlington-connected to said first transistor; a third transistor through which part of the base current of said second transistor flows; circuit means for supplying to said third transistor a base current from the terminal voltage of said motor; a fourth transistor to which part of the current flowing through said second transistor is applied as a base current; and a variable resistor through which both the current caused by the source voltage to flow through said fourth transistor and the current flowing through said third transistor flow, said variable resistor providing a reference voltage for setting the rotational speed of said motor to a predetermined value.

United States Patent 1 1 Watanabe Sept. 24, 1974 [75] Inventor: YoshiakiWatanabe, Kyoto, Japan [73] Assignee: Kabushiki Kaisha Morita Seisakusho, Kyoto, Japan [22] Filed: Jan. 2, 1972 [21] Appl. No.: 303,128

[30] Foreign Application Priority Data Primary Examiner-Bernard A. Gilheany Assistant ExaminerThomas Langer Attorney, Agent, or Firm-Christensen, OConnor, Garrison & Havelka [57] ABSTRACT A system for controlling the rotational speed of a micromotor for use in medical instruments such as a dental handpiece comprises a first transistor connected in series between said motor and a source of electric energy; a second transistor Darlingtonconnected to said first transistor; a third transistor 1971 Japan 46404806 through which part of the base current of said second transistor flows; circuit means for supplying to said [52] U.S. Cl. 318/331, 318/345 third transistor a base Curmnt from the terminal volt [51] Int. Cl. 1102p 5/16 I [58] Field of Search 318/331 332 345 age of said motor, a fourth transistor to which part of the current flowing through said second transistor is applied as a base current; and a variable resistor [56] References Clted through which both the current caused by the source UNITED STATES PATENTS voltage to flow through said fourth transistor and the 3,309,596 3/1967 Limley 318/345 current flowing through said third transistor flow, said 3,348,112 10/1967 S21E11 1 v 318/345 variable resist r providing a reference voltage for et- 3,422,331 1/1969 r r 318/345 ting the rotational speed of said motor to a predeter- 3,440,5ll 4/1969 lgarashl 318/331 mined value 3,566,229 2/1971 Wickens 318/332 3,617,846 11/1971 Hanberg 318/332 6 Claims, 1 Drawing Figure TRl o P VR 2 RLL i-Nl 2 m R3 PAIENIEUsEmmu MQM SYSTEM FOR CONTROLLING THE ROTATIONAL SPEED OF A MICROMOTOR FOR USE IN MEDICAL INSTRUMENTS This invention relates to a system for controlling the rotational speed of a micromotor for use in various medical instruments.

As is well known, micromotors are widely used as a drive for various instruments for cutting, grinding, forming, abraising and drilling of bones and tissues. Since the micromotor is very compact in size, it can be easily encased in an instrument having the shape of the elongated cylindrical body of a fountain pen, so that the doctor can easily manipulate the instrument in treatment. In dentistry the handpiece is an example of an instrument which uses a micromotor as a power source. The handpiece is provided at its outer end with a cutting tool which is rotated for performing the required treatment on the tooth. Since the handpiece is held by the hand like a pen, it must have such a shape that it can be easily gripped or held by the hand.

To make the handpiece compact, a micromotor is used as a drive for rotating the cutting tool. However, for such specific use in medical instruments the micromotor must be such that its rotational speed can be freely changed as occasions demand. In some cases, when the motor is being rotated under no load, it must be abruptly stopped. Also it is required that under loaded conditions the motor should be run at a constant speed to ensure accuracy in cutting or other operations and efficiency of the treatment.

To control the rotational speed of the micromotor there have been proposed various methods or systems. For example, one method uses a constant voltage circuit; another depends on detection of the counter electromotive force and a third utilizes the generated voltage of a generator. However, in the first method in which a constant voltage circuit is used, under a heavyload, low-speed condition, that is when a low voltage is being applied to the motor, the rotational speed cannot be kept constant but is greatly decreased. In the second method, while the counter electromotive force is being detected, the input voltage cannot be applied to the motor. Therefore, the input voltage must be applied by means of a chopper, so that the input voltage necessarily becomes higher than the counter electromotive force and the motor is alternately speeded up and down by the variation of the applied voltage. This causes vibration to the instrument, making it unsuitable for use in accurate cutting or forming operation. In the third method in which a generator is used, if the generator is attached to the micromotor, the casing of the instrument becomes the larger or longer, so that it requires a greater muscular force of the operator with a resulting increase in his or her fatigue. In addition, the control circuit becomes complicated, and since the various 7 elements of the control circuit are enclosed in a box remote from the motor in the handpiece so as to be remotely operated by the hand or foot of the operator, the connecting cables or wires become thicker, thereby obstructing free and efficient performance of the doc tor in treatment.

Accordingly, it is one object of the invention to provide a system for controlling the rotational speed of a motor, which is capable of running the motor at a constant speed regardless of variation of the load thereon.

The invention will be explained in detail with reference to the accompanying Drawing illustrating one preferred embodiment thereof.

In the Drawing there is shown a motor M which is connected to direct voltage source terminals P through a transisotr TR]. The transistor TRl directly controls the power applied to the motor M. A transistor TR2 is connected in Darlington configuration to the base of the transistor TRl. The base current of the transistor TR2 is provided by the current from the terminals P flowing through a resistor R1. Part of this current is applied to the collector of a transistor TR3. The transistor TR3 is controlled by the base current which is determined by a current caused by the voltage applied to the motor to flow through a fixed resistor R3 and a variable resistor VRl. A current flowing through a pre-fixed resistor VR2 and a fixed resistor R2 is applied to the base of a transistor TR4, the collector current of which flows through the previously mentioned variable resistor VRl.

As will be understood from the above description, the transistor TRI controls the supply of power to the motor, the transistor TR2 functions as an amplifier, the transistor TR3 functions as a voltage comparator and the transistor TR4, as a current amplifier. The variable resistor VRl provides a reference voltage for setting the number of rotation of the motor; the resistor R3 detects the voltage applied to the motor and the resistor VR2 detects the current applied to the motor.

In operation, suppose that a direct current voltage is applied between the terminals P. A base current is applied to the transistor TR2 through the resistor R1, whereupon the transistor TR2 fires so that the transistor TRl receives a base current through the resistor VR2 and fires, whereupon the voltage applied to the terminals P is impressed on the motor M. At the same time, a base current is applied through the resistors VR2 and R2 to tire the transistor TR4, the collector current of which then flows through the resistor VRl, thereby inducing across the resistor a reference voltage ERl in accordance with the resistance value of the resistor. The voltage ERI charges a condenser C.

Under the condition, suppose that the terminal voltage of the motor M has become higher than the sum of the reference voltage ER] and the voltage drop in the base forward direction of the transistor TR3. Then, more of the base current of the transistor TR2 provided through the resistor R1 than before flows into the col-,

lector of the transistor TR3, so that the base current of the transistor TR2 decreases, with a resulting decrease in the collector current of the transistor TR! and consequently in the voltage applied to the motor M.

On the contrary, when the voltage applied to the motor has dropped, the collector current of the transistor TR3 decreases, so that the base current of the transistor TR2 increases, with a resulting increase in the collector current of the transistor TRl and consequently, in the voltage applied to the motor M.

In the above manner, the voltage applied to the motor M is kept at a constant level as determined by the reference voltage ERl set by the variable resistor VRl, so that the motor can be run at a constant speed regardless of variation of the load thereon.

Since the voltage applied to the motor while it is running depends on the reference voltage ERI, if the resistance value of the resistor VRI is changed to change the reference voltage ERI, the rotational speed of the motor can be changed, In other words, it is possible to control the rotational speed of the motor M by means of the variable resistor VRl.

Since the current through the motor without any load thereon does not depend upon the voltage but is kept substantially constant, the collector current of the transistor TR4 is kept substantially constant, so that it is possible to have a desired level of the reference voltage ERl by changing the resistance of the resistor VRl. Of course, the base current of the transistor TR3 as well as the collector current of the transistor TR4 flow through the resistor VRl. However, if the resistances of the resistors VR2, R1 and R2 and the direct current amplification factors of the transistors TR3 and TR4 are so selected that the base current of the transistor TR3 becomes far smaller than the collector current of the transistor TR4, the reference voltage ERl will practically depends on the collector current of the transistor TR4 alone.

Suppose that with the resistor VRl set to a certain resistance value, the rotational speed of the motor M has been reduced by a load connected thereto. Naturally, the current through the motor M increases, so that the collector current of the transistors TRl and TR2 increases. This causes the base and collector currents of the transistor TR4 to increase, so that the voltage across the variable resistor VRl becomes greater than the reference level ERl that was under the noload condition. As a result, the voltage applied to the motor M increases, thereby increasing the rotational speed thereof to be maintained at the required constant speed.

Thus, in accordance with the invention, the rotational speed of a micromotor can be kept constant regardless of variation of the load thereon, and the speed can easily be controlled by simply adjusting a variable resistor. The torque characteristic of the motor is greatly improved, so that the motor can be used as a reliable drive which ensures constant speed running of the rotating tool in medical instruments.

What I claim is:

l. A system for controlling the rotational speed of a micromotor which micromotor is responsive to a motor voltage applied to its terminals, comprising:

a first transistor connected between said micromotor and a power source, said first transistor controlling the supply of power to said motor;

a second transistor Darlington-connected to said first transistor so that upon firing of said second transistor, said first transistor fires, the firing of said second transistor establishing a second transistor collector voltage and the firing of said first transistor establishing a first transistor collector voltage;

a third transistor, to which is applied in the baseemitter path the motor voltage applied to said motor, which motor voltage establishes a third transistor base current;

a variable resistor coupled at one end thereof to the emitter of said third transistor, the voltage across said resistor being a variable reference voltage;

circuit means having a first current flowing therein for providing a base current for said second transistor, which second transistor base current is a portion of said first current, the remaining portion of said first current flowing to the collector of said third transistor, a change in the motor voltage applied to said motor relative to said variable reference voltage causing a corresponding change in the base current of said third transistor and consequently the base current and collector current of said second transistor, thereby changing the collector current of said first transistor, and hence the motor voltage applied to said motor; and

a fourth transistor, having a fourth transistor collector current when conducting, connected between said power source and said one end of said variable resistance, to the base of which fourth transistor is applied a portion of the collector current of said second transistor, said fourth transistor collector current changing accordingly as the load on said motor changes, with a resulting change in said variable rcference voltage, thereby resulting in a change in the rotational speed of said motor.

2. A circuit for controlling the rotational speed of a micromotor which micromotor is responsive to a motor voltage applied to its terminals, comprising:

current passage means coupled to one terminal of a two-terminal source of direct voltage and operative to provide said micromotor with a variable current;

means including a variable resistance, for establishing an automatically variable reference voltage; means for detecting the motor voltage applied to said micromotor;

means for comparing said detected voltage with said variable reference voltage;

first control means responsive to the magnitude of said variable current and connected to said comparing means for automatically controlling said variable reference voltage; and

second control means responsive to said comparing means for controlling said current passage means to provide said variable current and the motor voltage to said micromotor such that the speed of said micromotor remains constant under varying loads.

3. A circuit according to claim 2, wherein said first control means comprises a first transistor the emittercollector circuit of said first transistor being connected between said one terminal of said source of direct voltage and one end of said variable resistance, the other end of said variable resistance being coupled to the other terminal of said source of direct voltage, the base of said first transistor being connected to said second control means.

4. A circuit according to claim 3, wherein the base of said first transistor is coupled to said one terminal of said source of direct voltage through a given resistance.

5. A circuit according to claim 4, wherein said comparing means is a second transistor, the collector of said second transistor being coupled to said second control means and said one terminal of said voltage source, the emitter of said second transistor being coupled to said one side of said variable resistance and the base of said second transistor being coupled to said detecting means.

6. A circuit in accordance with claim 5, wherein said second control means is a third transistor, the collector of said third transistor being coupled to said one terminal of said source of direct voltage, and the base of said first transistor, the emitter of said third transistor being coupled to said current passage means, and the base of said third transistor being coupled to said collector of said second transistor. 

1. A system for controlling the rotational speed of a micromotor which micromotor is responsive to a motor voltage applied to its terminals, comprising: a first transistor connected between said micromotor and a power source, said first transistor controlling the supply of power to said motor; a second transistor Darlington-connected to said first transistor so that upon firing of said second transistor, said first transistor fires, the firing of said second transistor establishing a second transistor collector voltage and the firing of said first transistor establishing a first transistor collector voltage; a third transistor, to which is applied in the base-emitter path the motor voltage applied to said motor, which motor voltage establishes a third transistor base current; a variable resistor coupled at one end thereof to the emitter of said third transistor, the voltage across said resistor being a variable reference voltage; circuit means having a first current flowing therein for providing a base current for said second transistor, which second transistor base current is a portion of said first current, the remaining portion of said first current flowing to the collector of said third transistor, a change in the motor voltage applied to said motor relative to said variable reference voltage causing a corresponding change in the base current of said third transistor and Consequently the base current and collector current of said second transistor, thereby changing the collector current of said first transistor, and hence the motor voltage applied to said motor; and a fourth transistor, having a fourth transistor collector current when conducting, connected between said power source and said one end of said variable resistance, to the base of which fourth transistor is applied a portion of the collector current of said second transistor, said fourth transistor collector current changing accordingly as the load on said motor changes, with a resulting change in said variable reference voltage, thereby resulting in a change in the rotational speed of said motor.
 2. A circuit for controlling the rotational speed of a micromotor which micromotor is responsive to a motor voltage applied to its terminals, comprising: current passage means coupled to one terminal of a two-terminal source of direct voltage and operative to provide said micromotor with a variable current; means including a variable resistance, for establishing an automatically variable reference voltage; means for detecting the motor voltage applied to said micromotor; means for comparing said detected voltage with said variable reference voltage; first control means responsive to the magnitude of said variable current and connected to said comparing means for automatically controlling said variable reference voltage; and second control means responsive to said comparing means for controlling said current passage means to provide said variable current and the motor voltage to said micromotor such that the speed of said micromotor remains constant under varying loads.
 3. A circuit according to claim 2, wherein said first control means comprises a first transistor the emitter-collector circuit of said first transistor being connected between said one terminal of said source of direct voltage and one end of said variable resistance, the other end of said variable resistance being coupled to the other terminal of said source of direct voltage, the base of said first transistor being connected to said second control means.
 4. A circuit according to claim 3, wherein the base of said first transistor is coupled to said one terminal of said source of direct voltage through a given resistance.
 5. A circuit according to claim 4, wherein said comparing means is a second transistor, the collector of said second transistor being coupled to said second control means and said one terminal of said voltage source, the emitter of said second transistor being coupled to said one side of said variable resistance and the base of said second transistor being coupled to said detecting means.
 6. A circuit in accordance with claim 5, wherein said second control means is a third transistor, the collector of said third transistor being coupled to said one terminal of said source of direct voltage, and the base of said first transistor, the emitter of said third transistor being coupled to said current passage means, and the base of said third transistor being coupled to said collector of said second transistor. 